1. Field of the Invention
The present invention relates to a digital-to-analog converter (D/A converter). The D/A converter according to the present invention is used in, for example, an analog-to-digital conversion device of a step-by-step comparison type for processing signals in electronic circuit devices.
2. Description of the Prior Art
A prior art D/A converter comprising a resistor ladder network having a plurality of first resistors of resistance value R and a plurality of second resistors of resistance value 2R is illustrated in FIG. 1. In the D/A converter of FIG. 1, "n-1" first resistors of resistance value R are connected in series between a first terminal P.sub.1 and a second terminal P.sub.n, connection points P.sub.2, P.sub.3, . . . , P.sub.n-1 being formed between adjacent resistors of resistance value R. One terminal of each of the "n" second resistors of resistance value 2R is respectively connected with the first terminal P.sub.1, the connection points P.sub.2, P.sub.3, . . . , P.sub.n-1 and the second terminal P.sub.n. Another resistor of resistance value 2R is connected between the second terminal P.sub.n and a ground GR. The other terminals Q.sub.1, Q.sub.2, . . . , Q.sub.n of said "n" second resistors of resistance value 2R are respectively connected to fixed contacts of switching elements a.sub.1, a.sub.2, . . . , a.sub.n. Switchable contacts of each of the switching elements a.sub.1, a.sub.2, . . . , a.sub.n consist of a function "1" contact which is connected to a reference voltage source V.sub.ref and a function "0" contact which is connected with the ground GR. Semiconductor switching elements may be used for the switching elements a.sub.1, a.sub.2, . . . , a.sub.n. The switching elements a.sub.1, a.sub.2, . . . , a.sub.n are switched in accordance with digital input signals. A function "1" contact is closed when the applied digital signal is signal "1", while a function "0" contact is closed when the applied digital signal is signal "0". An output analog signal V is produced at a terminal X as a result of a digital to analog conversion effected by the resistor ladder network containing the switching elements. The value of V is expressed as follows. ##EQU1##
In this equation (1), the value of each of a.sub.1, a.sub.2, . . . , a.sub.n is "1" when the switching element in question is connected to the reference voltage V.sub.ref, and "0" when the switching element in question is connected to ground GR.
The output impedance Z.sub.out at the output terminal X of the D/A converter of FIG. 1 is equal to R. The full-scale value V.sub.f of the output voltage of the D/A converter of FIG. 1 is obtained when all of the switching elements a.sub.1, a.sub.2, . . . , a.sub.n are in the "1" state, i.e. the value of the input signal is the maximum value. The value V.sub.f is expressed as follows. ##EQU2##
In the D/A converter of FIG. 1, it is necessary to vary the full-scale value V.sub.f depending on the use of the D/A converter. In the prior art, the full-scale value V.sub.f is varied in an analog manner.
However, such a prior art device is disadvantageous because another D/A converter must be provided to produce a variable reference voltage signal V.sub.ref which is supplied to the D/A converter of FIG. 1 in order to effect analog control of the full-scale value V.sub.f, and hence this prior art D/A converter device is very complicated.
Another prior art D/A converter of the resistor ladder network type is illustrated in FIG. 2. One terminal of each of a sequence of resistors of n resistance value R, 2R, 4R, . . . , 2.sup.n-2 R and 2.sup.n-1 R are joined together at the terminal P'. The other terminal of each of the sequence of n resistors is connected with the fixed contact of each of the switching elements a.sub.1, a.sub.2, . . . , a.sub.n. The function "0" contact of each of the switching elements is connected to ground GR, while the function "1" contact of each of the switching elements is connected to the reference voltage source V.sub.ref. The joined terminal P' of the resistors is connected to one input of a current-to-voltage conversion amplifier AMP. The other input of the amplifier AMP is connected to ground GR via a resistor Rg. A feedback resistor R.sub.0 is connected across the amplifier AMP. The current-to-voltage conversion ratio is varied by switching from the feedback resistor R.sub.0 to the feedback resistor R.sub.0 '. Thus, the full-scale value of the D/A converter of FIG. 2 is controlled by selecting the resistance of the feedback resistor R.sub.0 or R.sub.0 ' connected across the amplifier AMP.
The value of the output voltage V is expressed as follows. ##EQU3##
However, this prior art device is disadvantageous since it is difficult to carry out the automatic switching between the feedback resistors R.sub.0 and R.sub.0 ' using a digital signal, because it is necessary to change the connection of the connecting conductor W which connects the output of the AMP to one of the feedback resistors R.sub.0 and R.sub.0 '. This method of controlling the full-scale value of the D/A converter of FIG. 2 is time-consuming and inconvenient.
Examples of prior art D/A converters of the resistor ladder network type are disclosed in the following publications:
(1) FERRANTI Semiconductors, Integrated Circuits Data Book: "D-A/A-D Converter ZN 425", Page 28, FIG. 2. PA0 (2) DATA ACQUISITION PRODUCTS Catalog: "CMOS 10-Bit, Buffered Multiplying D/A Converter", Page 309, FIG. 1.